Off the shelf in the UK, they're sold in packs of either 8 or 16 tablets, and shops are legally only allowed to sell you two packs at once. Pharmacies can of course supply larger quantities with a prescription.

The recommended dose is one or two 500mg tablets every 4 hours with a maximum of 8 tablets per day.

Yeah, I think this is what's causing the confusion here. We don't measure pills in "packets" in the US, so I think some people are taking "4 packets" to mean that they only took four pills and of course that's absurd.

Asking the real questions.

I think in UK paracetamol is sold over the counter in 28 x 500mg tablets most commonly. Larger packs or higher strengths are prescription only I think.

So 1 pack should last 3.5 days at the max adult dose of 8 per 24h - but I think it'd recommend seeing your doctor if you get into a second pack and you're still using that maximum dosage.

Generally shops will limit sales to 2 packs, but it's easy to shop around so that's no limit.